1. Field of the Invention
This invention relates to a method of and apparatus for image forming suited for electrophotographic reproducing and electrostatic recording.
2. Description of the Prior Art
In a scanning exposure type electrophotographic reproducing machine, for instance, a document being reproduced is mounted on the document glass plate and, when the reproduction button is pressed, the exposure lamp together with the optical system including a reflecting mirror is caused to travel in a predetermined direction while the lamp is throwing light on the document. The reflected light corresponding to the shade of the document is irradiated through the optical system on the image retainer (for instance, a photosensitive drum) uniformly charged, whereby an electrostatic latent image is formed on the image retainer. Moreover, there is formed by developer a toner image corresponding to the shade of the document on the photosensitive drum. A recording medium (for instance, copying paper) is fed from the paper feeder in such a manner that it positionally matches the toner image on the toner image retainer before being allowed to tough the toner image retainer. The toner image formed on the surface of the image retainer is transferred to the copying paper by the transfer electrode. In the meanwhile, the image retainer is kept rotating in a predetermined direction and the toner image is bit by bit transferred to the transfer paper. Then the transfer paper carrying the toner image is separated from the toner image retainer before being sent to the roller fixing device. The roller fixing device comprising two rollers, at least one of which being a heated one, is used to fix the image formed by the developer on the copying paper with heat. Then the transfer paper is discharged from the reproducing machine and, after a visible image has been transferred thereto, the image retainer is cleaned so that excessive toner powder may be removed. This operation is repeated each time reproduction is made.
Although such a reproducing machine is used to reproduce a monochromatic images, there has also been proposed a reproducing machine capable of providing color copies as shown in FIG. 1.
The reproducing machine shown in FIG. 1 is theoretically not so much different from what is used to reproduce monochromatic images. In the case of a monochromatic-image reproducing machine, an exposure lamp is used to throw light on a document and the light reflected therefrom in proportion to the shade of the document is irradiated on a toner image retainer through an optical system inclusive of an reflecting mirror to form an electrostatic latent image on the image retainer, whereas the light reflected from a document in the case of a color reproducing machine is separated by a filter in order to take out monochromatic light, whereby only the light that has passed through the filter is thrown on the toner image retainer. In the color reproducing machine shown in FIG. 1, there is provided a filter back 2 containing three kinds of filters having different colors, so that different monochromatic light can be obtained by each filter from the light reflected from the document. For instance, the green filter is used to separate a color first and an electrostatic latent image is formed on the image retainer by an exposure device 12 using the light passed through that filter before being developed by a developing device 3B with magenta toner stored in a developing device 3. In this case, a visible image is formed by the magenta toner on the image retainer 1. The visible image is carried from a paper feeder box 8 and transferred by a transfer electrode 10 to transfer paper 7 wound on a transfer drum 4.
The image retainer 1 is freed from being charged by a charge eliminating electrode 11 after the transference and, after the remaining toner is removed by a cleaning device 5, the retainer is again charged by a charging electrode 9. The light passed through the blue filter is used to expose the image this time, which is developed by a developing device 3A containing yellow toner, whereby a yellow visible image is formed on the image retainer 1 before being transferred to the copying paper 7 in the same manner as above. Subsequently, the light passed through the red filter is used to expose the image, which is developed by a developing device 3C containing cyan toner, whereby a cyan visible image is formed on the image retainer 1 before being transferred to the transfer paper 7.
The transfer paper 7 is made to wind on the transfer drum 4 until all the colored visible images are completely transferred and sent to a fixing device 6 after the transfer and then discharged after fixing.
As set forth above, the color reproducing machine operates to produce copies in color by separating the light reflected from a document to obtain monochromatic light using filters, developing electrostatic images formed of monochromatic light by means of a developing device containing toner colored in a tint corresponding to each light color and repeating the transfer of the images to copying paper.
Although such a color reproducing machine is capable of producing color copies, it has posed the following problems:
(1) When it is desired to obtain a copy in a given color other than those of the developers, the colored toner of each developer must be transferred successively to copying paper on the transfer drum (4 of FIG. 1) and therefore transfer timing must be taken into consideration for each color. Accordingly, such operation tends to cause misalignment when registering.
(2) In addition to the problem of being out of register, the aforesaid operation may also cause resolution to be reduced due to color balance image processing.
(3) Since the charging.fwdarw.exposing.fwdarw.developing.fwdarw.transferring operation must be repeated for each color, it must be controlled accurately, and this increases copying time.
(4) At least three, or four if black is needed, units of developing means corresponding to the three primary colors together with a transfer drum 4 are required. Morover, the developing means and the transfer drums 4 must be arranged on the peripheral surface of the image retainer 1 in view of their operational roles. Consequently, some space is needed near the peripheral surface of the image retainer 1 for arranging the aforementioned parts; in other words, a color reproducing machine is necessarily larger than a monochromatic reproducing machine.
In addition to the problems enumerated above, any conventional color reproducing machine has the disadvantages of a limited choice of tones and combinations of colors.